1. Field of the Invention
The present invention relates to active material powder including an active material and lithium niobate attached to at least part of a surface of the active material, and a method of producing the same.
2. Description of Related Art
A metal ion secondary battery (for example, a lithium ion secondary battery; hereinafter also referred to as “all-solid-state battery”) including a solid electrolyte layer in which a flame-retardant solid electrolyte is used has advantageous effects in that, for example, a system for securing safety can be easily simplified.
As a technique regarding such an all-solid-state battery, for example, International Publication WO 2007/004590 discloses a technique of forming a LiNbO3 coating layer on a surface of a LiCoO2 powder particle through a step of hydrolyzing an alkoxide solution containing lithium and niobium on the surface of the LiCoO2 powder particle. In addition, International Publication WO 2007/004590 also discloses a technique of spraying the alkoxide solution on the surface of the LiCoO2 powder particle, hydrolyzing the alkoxide solution at an atmospheric humidity to form a coating layer, and then heating the coating layer at 400° C. for 30 minutes to obtain LiCoO2 powder having a surface coated with LiNbO3.
In the technique disclosed in International Publication WO 2007/004590, a LiNbO3 coating layer is formed on a surface of a positive electrode active material. Therefore, for example, when this positive electrode active material is used in combination with a sulfide-based solid electrolyte, a lithium ion conductive oxide layer can be allowed to be present at an interface between the positive electrode active material and the sulfide-based solid electrolyte. As a result, output characteristics of the all-solid-state battery can be expected to be improved. However, in a lithium ion secondary battery obtained using the technique disclosed in International Publication WO 2007/004590, an active material powder in which a LiNbO3 layer is foamed on a surface of an active material is used, and the active material (hereinafter, also referred to as “high-potential active material”) stores and releases lithium ions at a potential of 4.5 V or higher when a Li+/Li equilibrium potential is 0 V. In this lithium ion secondary battery, there is a problem in that a resistance after being repeatedly charged and discharged is likely to increase. That is, when the LiNbO3 layer is formed on the surface of the high-potential active material using the technique disclosed in International Publication WO 2007/004590, charging-discharging cycle characteristics are likely to decrease.